1. Field of the Invention
Embodiments of the present invention relate to seeding and/or making of dry hydrates and avoiding wax deposition without the aid of chemicals and with minimum use of rotating or other energized equipment. Other embodiments relate to the prevention of hydrate agglomeration and the prevention of wax deposition in a pipeline. The invention also relates to elimination of the use of energized equipment for melting, grinding or scraping hydrate solids and deposited waxes from inside of pipelines or flowlines. Also eliminated is the need for any recycle loops. In yet another embodiment there is no need for splitting the wellstream into two streams. In another aspect, the invention also avoids the use of rotating or other mechanized equipment that require remote vehicle intervention for maintenance and repair in subsea operations. In addition, embodiments of the invention eliminate the need for dual flowlines. Still other embodiments relate to the elimination of the need for heating or insulating flowlines for hydrate prevention and wax deposition prevention, thus reducing the cost of flowlines.
2. Discussion of Background Information
Among the most challenging problems in oil and gas production is the presence of natural gas hydrates in transport pipelines and equipment. Also very problematic is wax deposition in flow lines. Natural gas hydrate is an ice-like compound consisting of light hydrocarbon molecules encapsulated in an otherwise unstable water crystal structure. These hydrates form at high pressures and low temperatures wherever a suitable gas and water are present. Such conditions are prevalent in “cold-flow” pipelines, where the pipeline and wellstream fluids are unheated, and the wellstream fluids are allowed to flow through the pipeline at the low ambient temperatures often found in subsea environments. Cold-flow delivery of wellstream fluids is highly desirable, however, since it avoids the cost of insulating the pipeline and heating the pipeline and the contained fluids, but gas hydrate crystals can deposit on cold-flow pipeline walls and in associated equipment, and in the worst case lead to complete plugging of the system. Costly and time-consuming procedures may be needed to restore flow again in a pipeline plugged with hydrates and/or wax. In addition to the mere economic consequences, there are also numerous hazards connected to hydrate formation and removal, and there are known instances of pipeline ruptures and loss of human lives due to gas hydrates in pipelines. Although hydrate is generally thought of as a problem mostly for gas production, there is now ample evidence that it is also a significant problem for condensate and oil production systems. Wax deposition is also a costly problem when produced fluids naturally contain wax compounds, usually paraffin, that coat flow lines during liquid hydrocarbon production.
Several methods are known to prevent or eliminate hydrate formation and wax deposition, and subsequent problems in pipelines, valves and other production equipment, such as, for example, the processes disclosed in U.S. Patent Publication Nos. 20040176650 and 20040129609, U.S. Pat. No. 6,656,366. The article entitled “Continuous Gas Hydrate Formation Process by Static Mixing of Fluids,” Paper #1010 in 5th International Conference on Gas Hydrates, Trondheim, Norway, Jun. 13-16, 2005, by Tajima et al. contains additional background information.
Current methods of preventing or eliminating hydrate plug formation using dry hydrates may involve, at a minimum, a recycle loop of dry hydrates comprising a pump and/or grinder. In such methods, the continuous recycling of even dry hydrates in a recycle loop leads to the continued growth of the hydrates and the formation of larger and larger hydrates that, if not continuously ground into smaller hydrates using a grinder or similar equipment, would ultimately grow large enough to cause plugging. Unfortunately, the pump or grinder is an energized piece of rotating equipment that can pose problems in subsea applications. There are two problems with such subsea electrical rotating equipment. First, the reliability of rotating equipment is not yet sufficient to plan for long-term operation without multiple equipment replacements during the typical lifetime of a subsea pipeline. Second, electrical power transmission is limited in distance, thus limiting the distance over which some cold flow processes are useful.
Besides the problems of energized, rotating equipment in subsea applications, other problems occur with current cold flow methods, such as fluids forming “sticky hydrates”. If an unplanned shut-in occurs during the process, the reactor and possibly the main pipeline could experience a complete hydrate plug.
Some proposed solutions for generating dry hydrates for cold flow include rotating equipment, such as a pump or grinder. For example, the following have been proposed: the use of a modified pig with special pressure cleaning devices; subsea pig replacement devices operated by remote operated vehicles; high velocity, high-shear devices; mechanical scraping devices, including a rotating internal vane; near sonic pressure waves; and water hammer.
Many of the prior art methods use equipment that is not commercially proven and some of them require electricity. In addition, many require maintenance that is particularly costly in subsea applications.
Thus, there is a need for improved methods of seeding and/or making dry hydrates without the aid of continuous injection of chemicals and with minimum use of rotating or other energized equipment.
Wax deposition depends on the content of the produced or transferred fluid but usually occurs after production when the right temperature and pressure conditions are reached.